Staphylococcus aureus proteins Sbi and Efb recruit human plasmin to degrade complement C3 and C3b

Upon host infection, the human pathogenic microbe Staphylococcus aureus (S. aureus) immediately faces innate immune reactions such as the activated complement system. Here, a novel innate immune evasion strategy of S. aureus is described. The staphylococcal proteins surface immunoglobulin-binding protein (Sbi) and extracellular fibrinogen-binding protein (Efb) bind C3/C3b simultaneously with plasminogen. Bound plasminogen is converted by bacterial activator staphylokinase or by host-specific urokinase-type plasminogen activator to plasmin, which in turn leads to degradation of complement C3 and C3b. Efb and to a lesser extend Sbi enhance plasmin cleavage of C3/C3b, an effect which is explained by a conformational change in C3/C3b induced by Sbi and Efb. Furthermore, bound plasmin also degrades C3a, which exerts anaphylatoxic and antimicrobial activities. Thus, S. aureus Sbi and Efb comprise platforms to recruit plasmin(ogen) together with C3 and its activation product C3b for efficient degradation of these complement components in the local microbial environment and to protect S. aureus from host innate immune reactions.

Complement, Candida, and cytokines: the role of C5a in host response to fungi

Complement is the central host defense system that clears invading microbes and balances homeostasis. Pathogenic microbes such as Candida albicans have to breach this efficient and important immune defense layer in order to propagate within the host and to establish an infection. Knowing exactly how the activated complement cascade responds to and attacks microbial invaders is central to understanding the immune battle and the infection process. This also allows a better understanding of how Candida counteracts the individual steps of host innate immunity. Ultimately this knowledge will allow the design of appropriate therapeutic molecules. In this issue Cheng et al. [Eur. J. Immunol. 2012. 42: 993-1004] identify a new cellular effect of the activated human complement system in the defense against the fungal pathogen C. albicans. The authors show that the complement activation fragment C5a, which is formed in response to Candida infection, induces the cellular release of the inflammatory cytokines IL-6 and IL-1β.

Complement factor H binds malondialdehyde epitopes and protects from oxidative stress

Oxidative stress and enhanced lipid peroxidation are linked to many chronic inflammatory diseases, including age-related macular degeneration (AMD). AMD is the leading cause of blindness in Western societies, but its aetiology remains largely unknown. Malondialdehyde (MDA) is a common lipid peroxidation product that accumulates in many pathophysiological processes, including AMD. Here we identify complement factor H (CFH) as a major MDA-binding protein that can block both the uptake of MDA-modified proteins by macrophages and MDA-induced proinflammatory effects in vivo in mice. The CFH polymorphism H402, which is strongly associated with AMD, markedly reduces the ability of CFH to bind MDA, indicating a causal link to disease aetiology. Our findings provide important mechanistic insights into innate immune responses to oxidative stress, which may be exploited in the prevention of and therapy for AMD and other chronic inflammatory diseases.

The pH-regulated antigen 1 of Candida albicans binds the human complement inhibitor C4b-binding protein and mediates fungal complement evasion

Candida albicans binds and utilizes human complement inhibitors, such as C4b-binding protein (C4BP), Factor H, and FHL-1 for immune evasion. Here, we identify Candida pH-regulated antigen 1 (Pra1) as the first fungal C4BP-binding protein. Recombinant Pra1 binds C4BP, as shown by ELISA and isothermal titration calorimetry, and the Pra1-C4BP interaction is ionic in nature. The Pra1 binding domains within C4BP were localized to the complement control protein domain 4 (CCP4), CCP7, and CCP8. C4BP bound to Pra1 maintains complement-inhibitory activity. C4BP and Factor H bind simultaneously to Candida Pra1 and do not compete for binding at physiological levels. A Pra1-overexpressing C. albicans strain, which had about 2-fold Pra1 levels at the surface acquired also about 2-fold C4BP to the surface, compared with the wild type strain CAI4. A Pra1 knock-out strain showed ∼22% reduced C4BP binding. C4BP captured by C. albicans from human serum inhibits C4b and C3b surface deposition and also maintains cofactor activity. In summary, Candida Pra1 represents the first fungal C4BP-binding surface protein. Pra1, via binding to C4BP, mediates human complement control, thereby favoring the immune and complement evasion of C. albicans.

Complement factor H-related proteins CFHR2 and CFHR5 represent novel ligands for the infection-associated CRASP proteins of Borrelia burgdorferi

BACKGROUND

One virulence property of Borrelia burgdorferi is its resistance to innate immunity, in particular to complement-mediated killing. Serum-resistant B. burgdorferi express up to five distinct complement regulator-acquiring surface proteins (CRASP) which interact with complement regulator factor H (CFH) and factor H-like protein 1 (FHL1) or factor H-related protein 1 (CFHR1). In the present study we elucidate the role of the infection-associated CRASP-3 and CRASP-5 protein to serve as ligands for additional complement regulatory proteins as well as for complement resistance of B. burgdorferi.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate whether CRASP-5 and CRASP-3 interact with various human proteins, both borrelial proteins were immobilized on magnetic beads. Following incubation with human serum, bound proteins were eluted and separated by Glycine-SDS-PAGE. In addition to CFH and CFHR1, complement regulators CFHR2 and CFHR5 were identified as novel ligands for both borrelial proteins by employing MALDI-TOF. To further assess the contributions of CRASP-3 and CRASP-5 to complement resistance, a serum-sensitive B. garinii strain G1 which lacks all CFH-binding proteins was used as a valuable model for functional analyses. Both CRASPs expressed on the B. garinii outer surface bound CFH as well as CFHR1 and CFHR2 in ELISA. In contrast, live B. garinii bound CFHR1, CFHR2, and CFHR5 and only miniscute amounts of CFH as demonstrated by serum adsorption assays and FACS analyses. Further functional analysis revealed that upon NHS incubation, CRASP-3 or CRASP-5 expressing borreliae were killed by complement.

CONCLUSIONS/SIGNIFICANCE

In the absence of CFH and the presence of CFHR1, CFHR2 and CFHR5, assembly and integration of the membrane attack complex was not efficiently inhibited indicating that CFH in co-operation with CFHR1, CFHR2 and CFHR5 supports complement evasion of B. burgdorferi.

The autoimmune disease DEAP-hemolytic uremic syndrome

DEAP-HUS (deficiency of CFHR plasma proteins and factor H [FH] autoantibody positive hemolytic uremic syndrome [HUS]) is a new form of HUS characterized by a deletion of genes coding for FH-related proteins and the presence of autoantibodies directed to FH. These disease-associated autoantibodies inhibit FH (CFH) surface binding functions, which results in a defective regulation of the alternative pathway and damage of endothelial cells. Here we describe two representative patients with DEAP-HUS who both developed end-stage renal failure with the background of homozygous deletion of CFHR1 and CFHR3 genes and the presence of FH autoantibodies. Based on the retrospective diagnosis of DEAP-HUS 2 to 12 months after the initial clinical presentation, subsequent immunosuppressive therapy was initiated. The autoantibody titers decreased, and the complement status of the patients improved, as indicated by increased C3 levels. Thus early diagnosis of DEAP-HUS and immunosuppressive treatments are important factors to treat this particular type of HUS.

An imbalance of human complement regulatory proteins CFHR1, CFHR3 and factor H influences risk for age-related macular degeneration (AMD)

A frequent deletion of complement factor H (CFH)-related genes CFHR3 and CFHR1 (ΔCFHR3/CFHR1) is considered to have a protective effect against age-related macular degeneration (AMD), although the underlying mechanism remains elusive. The deletion seems to be linked to one of the two protective CFH haplotypes which are both tagged by the protective allele of single nucleotide polymorphism rs2274700 (CFH:A473A). In a German cohort of 530 AMD patients, we now show that protection against AMD conferred by ΔCFHR3/CFHR1 is independent of the effects of rs2274700 and rs1061170 (CFH:Y402H). This suggests a functional role of CFHR1 and/or CFHR3 in disease pathogenesis. We therefore characterized the CFHR3 function and identified CFHR3 as a novel human complement regulator that inhibits C3 convertase activity. CFHR3 displays anti-inflammatory effects by blocking C5a generation and C5a-mediated chemoattraction of neutrophils. In addition, CFHR3 and CFHR1 compete with factor H for binding to the central complement component C3. Thus, deficiency of CFHR3 and CFHR1 results in a loss of complement control but enhances local regulation by factor H. Our findings allude to a critical balance between the complement regulators CFHR3, CFHR1 and factor H and further emphasize the central role of complement regulation in AMD pathology.

Complement regulator-acquiring surface protein 1 of Borrelia burgdorferi binds to human bone morphogenic protein 2, several extracellular matrix proteins, and plasminogen

Lyme disease-causing Borrelia burgdorferi spirochetes express up to 5 complement regulator-acquiring surface proteins (CRASPs). To better define how CRASP-1 contributes to infection, we aimed to identify novel CRASP-1-binding host proteins. Here, we identified a number of novel human CRASP-1-binding proteins, including bone morphogenic protein 2, collagen I, collagen III, collagen IV, fibronectin, laminin, and plasminogen. The plasminogen-binding regions were located in 2 separate regions of CRASP-1. Our results demonstrated that plasminogen-bound CRASP-1 can be converted to plasmin by the urokinase-type plasminogen activator and that proteolytically active plasmin cleaves the synthetic chromogenic substrate S-2251 and the natural substrate fibrinogen. In conclusion, CRASP-1 is a multifunctional protein of B. burgdorferi that binds to several human extracellular matrix proteins and plasminogen. These interactions may contribute to adhesion, bacterial colonization, and organ tropism and may allow dissemination of B. burgdorferi in the host.

The yeast Candida albicans evades human complement attack by secretion of aspartic proteases

Candida albicans, which represents one of the most important human pathogenic yeasts, is directly attacked by the host innate immune system upon infection. However this pathogen has developed multiple strategies to escape host immune defense. Here, we show that C. albicans secreted proteases interfere and inactivate host innate immune effector components, such as complement proteins. Secreted aspartic proteases (Saps) in the culture supernatant of C. albicans cells and also recombinant Sap1, Sap2 and Sap3 degrade host complement components C3b, C4b and C5 and also inhibit terminal complement complex (TCC) formation. This proteolytic activity is specific to the three recombinant and wild type Sap proteins. The triple knock out C. albicans strain Delta sap1-3 and also the non-pathogenic yeast S. cerevisiae lack such degrading activities. The complement inhibitory role of Sap1, Sap2 and Sap3 was confirmed in hemolysis assays with rabbit erythrocytes and normal human plasma. Secretion of complement degrading proteases provides a highly efficient complement defense response of this human pathogenic yeast that acts after the immediate acquisition of host complement regulators to the cell surface.

Complement regulators and inhibitory proteins

The complement system is important for cellular integrity and tissue homeostasis. Complement activation mediates the removal of microorganisms and the clearance of modified self cells, such as apoptotic cells. Complement regulators control the spontaneously activated complement cascade and any disturbances in this delicate balance can result in damage to tissues and in autoimmune disease. Therefore, insights into the mechanisms of complement regulation are crucial for understanding disease pathology and for enabling the development of diagnostic tools and therapies for complement-associated diseases.

The complement fitness factor H: role in human diseases and for immune escape of pathogens, like pneumococci

Factor H is the central regulator of the alternative complement pathway and controls early activation of the complement cascade at the level of the C3 convertase. Mutations in the Factor H gene are associated with severe and diverse diseases including the rare renal disorders hemolytic uremic syndrome (HUS) and membranoproliferative glomerulonephritis (MPGN) also termed dense deposit disease (DDD), as well as the more frequent retinal disease age related macular degeneration (AMD). In addition, pathogenic microbes utilize host complement Factor H for immune evasion and these pathogens express specific surface receptors which bind host innate immune regulators. Sequence variations or mutations of one single gene, coding for the host regulator Factor H, form the basis for multiple, different disorders such as human renal and retinal diseases as well as infections. This association of Factor H but also of additional related complement components and regulators with the same diseases demonstrate an important role of complement, particularly of the alternative pathway, for tissue homeostasis. Disturbances of this central immune surveillance system lead to damage of autologous tissues and surfaces and result in autoimmune diseases. Remarkably, pathogenic microbes copy this mechanism of immune surveillance: they mimic the composition of host cell's, bind Factor H to their surface and engage acquired host Factor H for immune disguise.

Autoantibodies in haemolytic uraemic syndrome (HUS)

Haemolytic uraemic syndrome (HUS) is a severe disease with renal failure, microangiopathic anemia and thrombocytopenia. Several mechanisms leading to HUS have been identified, like infections with enterohaemorrhagic Escherichia coli, as well as genetic mutations of complement genes, which result in defective complement control on the surface of host cells. The complement system forms the first defense line of innate immunity and mediates the attack against foreign microorganisms. Defective regulation of this cascade results in attack of self cells and in autoimmune disease. Apparently, the alternative pathway convertase C3bBb is central for the pathophysiology of HUS as gene mutations of the components (C3 and Factor B) or of regulators (Factor H, Factor I and MCP/CD46) are observed in the genetic form of HUS. Recently, a novel mechanism leading to atypical HUS (aHUS) was identified, in form of autoantibodies that bind the complement inhibitor Factor H. Here we summarize the current concept of HUS and focus in particular on the novel subgroup of aHUS patients with IgG autoantibodies to Factor H which develop on the genetic background of CFHR1/CFHR3 deficiency, and which define a new subform termed DEAP-HUS (deficient for CFHR proteins and Factor H autoantibody positive).

The Staphylococcus aureus protein Sbi acts as a complement inhibitor and forms a tripartite complex with host complement Factor H and C3b

The Gram-positive bacterium Staphylococcus aureus, similar to other pathogens, binds human complement regulators Factor H and Factor H related protein 1 (FHR-1) from human serum. Here we identify the secreted protein Sbi (Staphylococcus aureus binder of IgG) as a ligand that interacts with Factor H by a—to our knowledge—new type of interaction. Factor H binds to Sbi in combination with C3b or C3d, and forms tripartite Sbi∶C3∶Factor H complexes. Apparently, the type of C3 influences the stability of the complex; surface plasmon resonance studies revealed a higher stability of C3d complexed to Sbi, as compared to C3b or C3. As part of this tripartite complex, Factor H is functionally active and displays complement regulatory activity. Sbi, by recruiting Factor H and C3b, acts as a potent complement inhibitor, and inhibits alternative pathway-mediated lyses of rabbit erythrocytes by human serum and sera of other species. Thus, Sbi is a multifunctional bacterial protein, which binds host complement components Factor H and C3 as well as IgG and β₂-glycoprotein I and interferes with innate immune recognition.

Staphylococcus aureus is a Gram-positive bacterium that can live as a commensal but can also cause severe life threatening infections in humans. Upon infection the bacterium is attacked by the host immune system, and in particular by the complement system which forms the immediate, first defence line of innate immunity. In order to survive, S. aureus has developed multiple evasion strategies and uses several virulence factors to evade and inactivate the host complement attack. Here we show that this pathogen binds the host complement regulators Factor H from human serum with the secreted and surface exposed Sbi protein, by a—to our knowledge—new type of interaction. Factor H binds to Sbi in combination with another host complement protein C3, C3b or C3d, and forms tripartite Sbi∶C3∶Factor H complexes. As part of this tripartite complex, Factor H is functionally active and inhibits further complement activation. Sbi, by recruiting Factor H and C3b, acts as a potent complement inhibitor, and inhibits alternative pathway-mediated lyses of rabbit erythrocytes by human serum and sera of different species. Thus, Sbi is a multifunctional bacterial protein, which binds host complement components Factor H and C3 as well as IgG and β₂-glycoprotein I and interferes with innate immune recognition.